Nobody thinks about batteries—until they've run out of juice, of course. But this humble and surprisingly ancient technology has done far more for human civilization than most people realize.

In fact, the modern world as we know it wouldn't exist were it not for batteries and the unique, utterly essential ability to store electrical energy that they provide. Without batteries, there is no such thing as mobile. Phones, computers, bio-medical devices, even the lowly flashlight: every single electronic device on Earth would have to compete for open an wall socket just to turn on.

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So let's think about batteries for a minute. Or better yet, explain everything you could possibly want to know about what they are, where they come from, and—most importantly—how to get the most out of them.

A Brief History of Batteries

Baghdad Battery: Bringing Bling to Mesopotamia
It's like a scene from Raiders of the Lost Ark. In 1936, a number of small, oddly-anointed terracotta pots were discovered in in the ruins of a village near the modern-day town of Khuyut Rabbou'a on the outskirts of Baghdad, Iraq. Clearly from antiquity, their ages dated to either the Parthian era (248 BC - 226 AD) or Sassanid (224-640 AD), but without an obvious use, the curious jars were donated to the National Museum of Iraq. There they sat on a shelf for two years until the museum's German director, Wilhelm König, rediscovered them in the museum's archives around 1938. Upon König's return to the fatherland in 1940 due to illness, he published a speculative paper that the mysterious vessels may have been a lost precursor of the galvanic cell, one perhaps used to electroplate thin layers of gold onto the plated silver pieces he kept finding during excavations.

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Each clay jar measured roughly 5.5 inches tall and was outfitted with a small copper tube (constructed from a rolled copper sheet) surrounding an oxidized iron rod but separated by an asphalt plug. Were the vessel to be filled with an acidic or alkaline liquid, say, lemon juice, grape juice, or vinegar, many experts believe that it could well have generated a small but appreciable current (on the range of .8 to 2 volts if replica devices are any indication). Granted, this amount of voltage generally isn't powerful enough for the uses König imagined as the Mythbusters proved. Anthropologists now believe those pieces were fire-plated using mercury and some speculate that the Baghdad Battery could instead have served as a miracle device in ancient religious or healing ceremonies.

"The batteries have always attracted interest as curios," Dr Paul Craddock, a metallurgy expert of the ancient Near East from the British Museum, told the BBC. "They are a one-off. As far as we know, nobody else has found anything like these. They are odd things; they are one of life's enigmas."

Frog's legs are funny things. Not just good eatins, they exhibit a tendency to flail when exposed to an electrical charge. At least, that's what Luigi Galvani discovered in 1771 as a professor at the University of Bologna. As the legend goes, he was in the process of skinning a frog pinned via copper hooks to a table where he had just previously conducted various static electric experiments. Galvani's assistant picked up a metal scalpel from the table (which, unbeknownst to either man, carried a static electric charge) and accidentally touched an exposed sciatic nerve. With a small spark, the leg twitched and Galvani glimpsed that electric charge could actually transported by ions, not through fluids or the atmosphere as earlier theories posited.

He didn't actually figure that out, mind you, he incorrectly assumed that this "animal electricity" originated in the tissue itself, conducted by an "electrical fluid." This perverse notion would pervade for nearly thirty years, Galvani's discovery that two metals, when connected via a salt bridge and simultaneously touched to a nerve would cause such a reaction paved the way for the modern electric battery and the advent of the Galvanic cell.

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Voltaic Pile: Were Yurtle the Turtle a Galvanic Cell

Galvani's misconceptions about the origin of electricity in living beings lasted for the rest of his life, believing that "animal electricity" was borne from a muscle within the hip. Alessandro Giuseppe Antonio Volta, professor of experimental physics at the University of Pavia, was one of the first of Galvani's contemporaries to recreate his famous frog experiment and originally held the same views on animal electricity's hip-based origins. But what Volta realized, and Galvani did not, was that the frog leg was both capable of conducting and detecting electricity.

To prove this point, Volta built a a device he dubbed an "artificial electrical organ." Created by alternatively stacking silver and zinc discs separated by brine-soaked cloth, this arrangement created a circuit and would conduct a charge when connected by a wire. As Volta wrote to Sir Joseph Banks, the president of the Royal Society of London, on the 20th of March 1800:

... In this manner I continue coupling a plate of silver with one of zinc, and always in the same order, that is to say, the silver below and the zinc above it, or vice versa, according as I have begun, and interpose between each of those couples a moistened disk. I continue to form, of several of this stories, a column as high as possible without any danger of its falling.

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While he had been attempting to mimic a perceived biological function, Volta had actually invented the Voltaic Pile, world's first electric battery. It also led him to discover Volta's Law of the electrochemical series—the electromagnetic force of a galvanic cell is dependent on the electrical potential difference of the electrodes. This is why stacking nothing but copper or silver discs will not generate a current.

Galvanic cells rely on a pair of half-cells, each with a differing electrode dipped in electrolytic solution to generate a RedOx reaction and, in turn, an electrical charge. Unlike an electrolytic cell that requires electrical input to get started, the galvanic cell's RedOx reaction is spontaneous. That is, it occurs without any outside impetus. As such, galvanic cells were originally found use powering telegraph lines and improved designs are now often found in batteries, pH meters, and fuel cells.

The Voltaic pile didn't just lead to batteries. It's invention directly led to numerous other major scientific discoveries. William Nicholson and his half-brother Anthony Carlisle built their own voltaic pile and passed the current through a trough of water, thereby discovering electrolysis as the H20 decomposed into its constituent elements. Humphry Davy in the UK used his own voltaic pile to demonstrate that Volta's Law was based on a chemical reaction, not just the electrode's difference in potential, as well as further Nicholson's and Carlisle's electrolysis work. William Hyde Wollaston proved that voltaic and static electricity were one and the same. And just two years after its creation, Vasily Petrov was using them to study electrical arcs.

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While the Voltaic pile was a pioneering effort, it wasn't exactly practical—short circuits from leaking electrolyte were as common as hydrogen bubbles forming on the leads. These shortcomings spawned a raft of design improvements and re-imaginings. To solve the short circuiting issue, William Cruickshank, professor of chemistry at the Royal Military Academy, Woolwich, simply laid the pile on its side in an insulated rectangular box with pairs of zinc and copper plates welded together. These plates were evenly spaced throughout the box and created self-contained cells for the electrolyte mixture of dilute sulfuric acid.

The spillage issue was also addressed in 1812 with the invention of the Zamboni pile—also known as the Duluc Dry Pile—invented by Giuseppe Zamboni. Constructed from thousands of sheets of alternating silver and zinc foil, the Zamboni pile's manganese oxide electrolyte was held in place with honey and the entire affair was crammed into a long glass jar and insulated with pitch. This construction gives Zamboni piles potential outputs in the Kilowatt range, which makes modern versions very handy in a variety of military and scientific applications.

Perhaps the most momentous improvement upon Volta's original design came in 1866. Georges Leclanché found that a cell employing a zinc anode and manganese dioxide/carbon cathode dipped in a bath of ammonium chloride would produce 1.4 volts, a far cry from the original pile's .4V and equivalent to a modern alkaline battery. With a stronger electrical oomph, the dry Leclanché cell quickly found use powering early telephone systems. The only problem was that the chemical reaction that powered the phone also caused resistance to increase, causing the battery to quickly run down. The process would reverse when load was removed from the cell but you'd have to keep your conversations short. More importantly, Leclanché's cell laid the groundwork for the modern dry cell batteries that now power a majority of our gadgets.
[Smith University - Kenyon University - University of Houston - BBC - About - University of Wisconsin - Sparknotes - University of Rome - Wikipedia 1, 2, 3, 4, 5, 6 - Baghdad Battery Image: Smith University, Frogs Legs Image: Monkey Business Images / Shutterstock, ]

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How Many Different Kinds Are There?

Batteries on the whole are divided into two main classes: Primary cells, which are single-use disposables, and secondary cells, those that can be recharged. Primary cells are typically constructed from Alkaline, Zinc, or Lithium-based chemistry. They're relatively cheap to produce and are designed to be thrown away or recycled after dispensing their initial charge, however they are not exactly environmentally-sensitive. Secondary cells, on the other hand, are usually made from Lead-Acid, Nickel, Lithium-ion chemistry and tend to be more expensive up front than primaries. They can be reused over and over though, so are more economical and environmentally-friendly in the long run.

They're not divided simply by their reusability, mind you. Batteries can also be classified by their cell type into four main groups:

Wet cell: Wet cells are the oldest type of battery and use a liquid electrolyte to transport ions. Some of the ealiest examples were simply open-top glass jars filled with electrolyte solution and a pair of electrodes sunk into it. While eventually replaced by dry cells in most energy storage applications, wet cells are still widely employed as car batteries, in home chemistry sets, and by a variety of public utilities. They can be utilized as either primary or secondary cells.

Dry cell: Dry cells work like wet cells, except that the electrolyte solution exists in a paste form with just enough moisture to conduct a charge but not enough to go sloshing about when dropped. These cells, developed from the pioneering Leclanche cell, can also be used as either primary or secondary cells and are generally much safer to use than wets.

Molten salt: You won't find molten salt powering your wristwatch any time soon. These specialized industrial batteries rely on salt, super-heated to the point of liquification, as an electrolyte. As terrifying as driving around with liquid hot magma under your hood, a molten salt battery's energy density is quite high, making it a potential power source for electric vehicles.

Reserve: Reserves are all the battery with none of the electrolyte, which makes them perfect for short-term use after long-term storage. By separating the electrolyte from the rest of the battery assembly, the cell will not self-discharge while on the shelf. These are more commonly found in scientific and military applications than consumer.

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What all battery types have in common—from the tiny button cells that power watches to multi-ton blocks that back up the local telephone exchange—is that they're the most common example of voltaic electrochemical cells (well, technically, they're a collection of multiple cells working in unison, but still). An electrochemical cell is a device that produces an electrical charge from a chemical reaction. Each such cell comprises three primary elements: two electrodes—one positive, one negative—and an electrolyte solution to transport ions between them during a reduction-oxidation reaction that actually frees electrons from their atoms. If a cell utilizes two kinds of electrolyte, salt bridges may also be used to provide ionic contact while preventing the two solutions from mixing.

Reduction-Oxidation (RedOx) reactions are quite common throughout nature, including all chemical reactions wherein the oxidation state of an atom occurs (read: when electrons are transferred). This might be when the human body oxidizes glucose during the metabolic process, when carbon is reduced by hydrogen to create methane, or conversely, when methane reacts with oxygen oxidizing into carbon dioxide and water (The carbon is oxidized, giving electrons to the oxygen). RedOx reactions are essentially a pair of simultaneously-occurring half reactions that together form a whole reaction, similar to lead-acid reactions. During reduction, atoms gain an electron (lowering their oxidation state) and on the other hand, during oxidation, atoms lose electrons (increasing their oxidation state). In batteries, the two halves of the reaction are intentionally separated so that electrons generated at the positive electrode are forced to travel through a circuit—and power our gadgets—to reach the negative electrode.

With such a menagerie of cell types and applications, how can we accurately compare performance between batteries? The answer lies in the battery's capacity. Measured in milliamps per hour (mAh), a battery's capacity refers to its total available energy storage. So if a battery is rated at 2,500 mAh, as many AA alkaline batteries, it should deliver 2500 mA of energy for one hour. However comparing batteries on capacity alone only works when comparing batteries of equal size and composition. See, the chemistry of the battery and the type of device it is used in both play a significant role in how long a battery actually lasts. Alkaline-based primary batteries (ie. Duracell Copper tops) typically have higher capacity ratings (around 2500 mAh) but are far more likely to deliver their full rated capacity if the power is drawn slowly in devices like smoke alarms or children's toys. Lithium-based secondary batteries (ie. Duracell Stay Charged), on the other hand, are typically rated lower (2000 mAh) but perform better and last longer than their disposable counterparts when used in high-drain devices like digital cameras. The rule of thumb—your kids can get by with beat around disposables; keep the higher-priced rechargeables for your own toys.

Modern Era: Primary Batteries

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Alkaline batteries, by which the family of zinc-chemistry primaries is commonly known, are the most widely used in the world, accounting for a whopping 70 percent of the primary battery market in 2011 with 10 billion individual units produced worldwide and expected to rise in value to $5.4 billion in the US alone by 2015. Not bad considering alkalines have only been around since the 1950s.

Granted, primary cells that utilize alkalines rather than acids for their electrolyte solution have existed since the turn of the 20th Century thanks to the independent discoveries and work of both Waldemar Jungner and Thomas Edison. However, it is Canadian engineer Lewis Urry, a researcher for the Union Carbide's EverReady Battery division, that is credited with inventing the now-ubiquitous chemistry. In alkaline batteries, the anode is made of zinc powder (which increases its surface area and aids in conducting a charge while the cathode is made of manganese dioxide mixed with carbon. Alkaline differ from other zinc-anode batteries on account of, and get their name from, their unique electrolyte which uses potassium hydroxide rather than ammonium chloride or zinc chloride. This difference grants alkaline a higher energy density, longer service life, and longer shelf-life while delivering the same voltage—1.55 to 1.7V initially, declining to about 0.8V—as their acidic counterparts. In all, alkaline typically have a 3-5 year shelf lif compared with just 2-3 for the zincs.

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Zinc-Carbon and Zinc-Chloride are two such acid-based primaries. Zinc-carbon batteries have existed since 1886 when Dr. Carl Gassner patented a dry Leclanché cell utilizing a compressed block of manganese dioxide surrounded by a zinc can that also acted as the anode. For an electrolyte, Dr. Gassner dissolved ammonium chloride in water and mixed it the plaster of Paris to create a gel. Zinc-carbon batteries became the first commercial dry cell battery in 1900 when the EverReady battery company began selling them. They're still the least expensive primary battery to produce and are marketed for general use, however, they work best in low drain and intermittent use devices like remotes, flashlights, and clocks. Zinc-carbons are so cheap, they're still widely used as the batteries what come free with the gadget.

Between the three, the zinc carbons are by far the least powerful. While Zinc-chlorides, which replaces the ammonium chloride with a more conductive electrolyte for a steadier output, weigh 20 percent more than zinc-carbons, they offer 50 percent higher energy density, sustain high drain devices better, have a longer storage life of 2-3 years, and perform better at low temperatures. Alkalines are better than both of them, offering a 3-5 year shelf life, 500-700 percent energy capacity and operate without significant degradation in high temperatures.

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Alkalines themselves are outclassed by another form of primary battery chemistry: non-rechargable Lithium. These batteries are constructed with anodes of lithium or lithium–iron disulfide and a cathode of manganese dioxide suspended in an electrolyte paste of dissolved lithium salt. It's also sometimes known as "voltage-compatible" lithium, since this sort of battery can produce produce voltages from the 1.5V charge of AA or AAA alkalines up to 3.7v, though unfortunately, these batteries use a metallic form of lithium for their electrodes that precludes their recharging. You can find them sold under brand names like Energizer Lithium and Rayovac Lithium Photo. They offer more than double the operational life of alkalines in high discharge devices, thanks to their higher capacity and lower internal resistance, and suffer from virtually zero self-discharge which allows them a 10 year shelf life. They are, however more expensive than conventional alkalines and cannot be recharged like NiMH cells.
[PR Newswire - Wikipedia 1, 2, 3 - Energizer - eHow - Radioshack - Battery University - Image: f-f-f-f / Shutterstock, Vladimir Arndt / Shutterstock]

How to Get the Most Out of Your Primary Batteries

While primary batteries only have a single charge in them, there's a lot you can do to ensure you wring every last volt from them. Storing batteries in the refrigerator is one such method, perhaps the most well known. The lower temperature in the fridge reputedly slows the chemical reaction occurring within the battery, reducing the self-discharge rate, and extending the battery's shelf life by about five percent. Be sure to keep the batteries in a moisture proof bag to avoid corrosion on the leads and warm the batteries back to room temperature before using them.

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Battery manufacturers like Duracel and Energizer, however, frown on this practice citing potential corrosion or seal damage from the moisture without any additional battery life. Instead the company recommends storing batteries between 68 and 78 degrees F at 35 to 65 percent humidity. Under those conditions, alkalines should last between five and seven years, carbon zinc for three to five years, and lithium cells for 10 to 15 years on the shelf.

Conversely, primary batteries should never be stored at high temperatures above 80 degrees F as doing so will increase the self-discharge rate and drastically reduce their life expectancy. Nor should they, and this should go without saying, ever be recharged. Attempting to recharge a primary battery will likely cause it to fail and explode. And exploding batteries are about as fun to be around as exploding Xenomorphs.

Modern Era: Rechargables

Secondary batteries have existed for nearly as long as their single-serving counterparts but are capable of reversing their ion-producing chemical reaction by oxidizing the cathode and reducing the anode in the presence of a reverse current. In a word, they recharge when you reverse the flow of electrons in the circuit. The three most common chemistries of secondary batteries—lead-acid, nickel-based, and lithium-ion-based—all play ubiquitous roles in modern society, powering everything from cell phones to laptops to cars to server farms.

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Gaston Plante invented the oldest original rechargeable battery technology, the lead-acid wet cell, back in 1859. These batteries uses lead electrodes—one lead, one lead dioxide paste—submersed in a four mole 35/65 sulfuric acid/water concoction, known as electrolyte. Interestingly, unlike nickel- and lithium- based chemistry, the electrolyte in a lead-acid battery does more than just act as a buffer between the poles but actively contributes electrons to the process. As a lead-acid battery discharges current, the electrolyte and the plates oxidize, giving up electrons to produce water and lead sulfate. Reversing the current and dumping electricity back into the cell also reverses the reaction, producing electrolyte and lead, which reforms on the electrode.

Lead-acid batteries are not terribly efficient, in fact, they possess a staggeringly low energy capacity given their weight and volume. What they do have, however, is power. Lead-acid batteries are capable of delivering high voltage surges that other batteries cannot. This makes them a terrible choice for powering a Prius (just imagine all that sulfuric acid sloshing around, fun!) but terrific for jumping its starter motor when you turn the car on. That's why most of the automotive and marine batteries on the market today are of the lead-acid variety.

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Lead-acid batteries also excel at holding a charge, which makes them ideally suited for intermittent utilities like railroad crossing signals where a single battery may be used for up to 25 years. and where weight is not an issue—say, as emergency backups for hospitals, telecommunications centers, and server farms—lead-acid batteries are routinely employed given their long shelf-life. The US Navy even employs enormous lead-acid batteries to drive the electric propulsion of its modern nuclear submarines.

In the 1970s, this technology underwent a significant change with the advent of the valve regulated lead-acid (VRLA)——aka "sealed"—battery, which uses an electrolyte suspended in gel rather than liquid, allowing it to be used in any position much like the Duluc Dry Pile before it. They aren't literally sealed, mind you. See, when VRLA batteries discharge they generate oxygen at the positive pole which combines with hydrogen produced at the negative to create water. Which is great because it frees you from needing to top off the water of conventional flooded lead-acids. And if they produce too much hydrogen, the cells are equipped with pressure sensitive valves to vent the excess pressure (as much as 40 PSI on some designs) caused by overcharging. However, if these valves are faulty or blocked by debris, they can explode.
VRLA batteries come in two varieties, Absorbed Glass Mat (AGM) and Gel. AGM batteries suspends the electrolyte in a fiberglass mat separator with the plates close on either side to improve the battery's cycle efficiency. These batteries, while twice the price of a premium flooded lead-acid, are often found in deep-cycle applications, solar energy storage,and vehicle starting. Their lack of liquid electrolyte makes them uniquely suited for cold weather applications—Arctic ice monitoring stations, for instance—as they will not freeze and crack as wet cells do. Gel cells, on the other hand, have their electrolyte mixed with silica dust to form an immobile gel. This allows them to be used in more adverse environments and withstand more extreme low temperatures, shock, and vibration than wet cells. They also boast long shelf lives and provide better reliability compared with the flooded variety.

VRLA batteries do have some significant drawbacks as well. There's their price—AGM batteries run roughly double the retail price of a premium wet cell, on average, and Gel batteries can fetch five times as much. They also recharge more slowly and at a lower voltage than wet cells due to the presence of calcium on the plates to reduce water loss. Being "sealed," there is no way to check the electrolyte concentration. And, compared to wet cells, these batteries are much less tolerant of high temperatures and over charging, both of which significantly shorten the battery's operational life.

How to Get the Most Out of Your Lead-Acid Batteries

Lead-acid batteries in vehicles lead a hard life and rarely last more than five or six years under even ideal conditions and there are plenty of ways a lead-acid can fail. But, with some basic maintenance, you can make sure your battery lasts the duration of your car's lease.

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This sort of battery can suffer from corrosion of their external hardware—the cable ends, terminals, and connectors—caused by contact with electrolyte that has seeped from an over-filled wet cell (though hot weather can instigate the same effect due to natural fluid expansion) or by leaking acidic gasses. If you battery is serviceable (read: not sealed) take care to not overfill it and only use distilled water. If you notice a white or blue powdery substance forming on the battery connectors, disconnect the cell from the car's electrical system and clean the outside of the battery using a mixture of baking soda and water (two tablespoons per pint). After you set the battery back in its holder, be sure to re-secure it in place, then clean and tighten the cables ends and connectors leading to the terminals. A liberal application of high temperature grease or Vaseline coating the battery posts and cable ends will help minimize future corrosion.

Lead-acid batteries, for the most part, are designed to provide short bursts of high energy, not long periods of deep discharge cycles. The sulfate that forms on the battery's lead plates as it discharges (a process known as sulfation) is electrically insulating which increases the cells internal resistance and lowers its maximum current and shortens the battery's operational life. These batteries should be recharged whenever they drop to 70 percent capacity and never be allowed to drop below twenty percent, otherwise the heat generated from the increased resistance will damage the battery as it charges. To maximize the battery's life, especially if it is being stored for long periods of time (up to two years), you should routinely apply a "temperature-compensated float charge" to the battery using a maintenance charger.
If you've forgotten to check and recharge that stored battery for a few seasons, don't throw it away just yet. Applying an equalizing charge can reverse the sulfation process in some cases. As Battery University explains, you must first measure the charge of each individual cell:

One method is to apply a saturated charge and then to compare the specific gravity readings (SG) on the individual cells of a flooded lead acid battery. Only apply equalization if the SG difference between the cells is 0.030. During equalizing charge, check the changes in the SG reading every hour and disconnect the charge when the gravity no longer rises. This is the time when no further improvement is possible, and a continued charge would cause damage. The battery must be kept cool and under close observation for unusual heat rise and excessive venting. Some venting is normal and the hydrogen emitted is highly flammable. The battery room must have good ventilation. Equalizing VRLA and other sealed batteries involves guesswork. Good judgment plays a pivotal role when estimating the frequency and duration of the service. Some manufacturers recommend monthly equalization for 2 to 16 hours. Most VRLAs vent at 34kPa (5psi), and repeated venting leads to the depletion of the electrolyte that can lead to a dry-out condition.

Nickel Me This

The lead-acid wet cell reigned as the dominant—well, only—form of rechargeable battery for 40 years. In 1899, Swedish inventor, Waldemar Jungner, devised the Nickel-Cadmium cell. His Ni-Cd wet cells were more powerful and more sturdy than older lead-acid format. Unfortunately, very few people in America had ever heard of Jungner or his work (though he did eventually start his own battery company in 1906), so the discovery went widely unnoticed. It wasn't until 1902, when Thomas Edison created his own Nickel-cadmium cell, that the technology gained a foothold in the US.

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By 1946, Ni-Cd cell technology had evolved from the original "pocket type" cell, which dipped the electrodes into nickel-plated steel pockets, into the modern sintered plate, "jelly roll" style battery. Sinter plates are created by compressing nickel powder into a cohesive, fused disc using immense pressures rather than melting it. This produces an extremely porous material that can then be soaked in aqueous nickel and cadmium to create the cathode and anode. NiCd batteries come in a variety of shapes and sizes, vented and sealed, from tiny button batteries in hearing aids, photographic equipment, R/C models, cordless power tools, uninterrupted power supplies—even the world's largest battery in Fairbanks, Alaska. And, by rolling these two plates together with a dry-cell electrolyte paste between them, they can be used in AA and AAA battery form factors while providing a much higher maximum current and lower internal resistance than similar alkalines.

Ni-Cd batteries (not to be confused with "NiCad" batteries which are a registered trademark of SAFT Corporation) utilize nickel oxide hydroxide and metallic cadmium electrodes. These batteries discharge at a lower voltage than alkaline or zinc primaries—1.2 versus 1.5 volts, respectively—however, their chemistry allows them to operate in low temperatures or in high discharge applications without sacrificing cycle life and capacity. On the other hand, NiCd batteries are more expensive than lead-acid, have a higher self-discharge rate, which cuts down on their storage times, and may cause some devices designed specifically for alkaline batteries to not operate properly on account of the lower terminal voltage and smaller ampere-hour capacity. And while they provided much more power than a lead-acid, disposing of cadmium in an expensive and environmentally damaging proposition. This is a large part as to why they are quickly being supplanted by Nickel Metal Hydride batteries.

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Nickel-Metal Hydride batteries are very similar to their NiCd cousins in their construction with both using nickel oxyhydroxide cathodes and both producing a terminal voltage of 1.2V. NiMH batteries however replace the cadmium anode with one made from an hydrogen-absorbing alloy made from a mix of rare earths (either by combining lanthanum, cerium, neodymium, and praseodymium or by using an amalgamation of nickel, cobalt, manganese, and/or aluminium) and use an alkaline electrolyte, typically potassium hydroxide. This grants the NiMH both 200-300 percent the capacity of a NiCd and a current output close to that of a Lithium-ion cell. In some high-performance applications the anode may also be made from a titanium/zirconium compound which boost its capacity further.

Development on NiMH technology began in 1967 at the Battelle-Geneva Research Center (with significant funding from Daimler-Benz and Volkswagen) and went on for two decades before the first commercial NiMH battery hit the market in 1989. The technology also garnered significant interest in the 1970s as a means of cheaply and compactly storing hydrogen for use in satellites. From those disco-era designs grew the modern NiMH chemistry. Today, high-energy NiMH batteries power the world's two million or so hybrid vehicles, have completely replaced NiCd batteries in the EU for all consumer appliances, and are regularly used in high-drain, heavy-use applications that would smoke an average alkaline in seconds.

While NiMH cells hold up to 40 percent more charge, exhibit a weaker memory effect, relatively inexpensive, and more environmentally-friendly, they unfortunately self-discharge like mad. A stored NiMH battery may lose as much as 20 percent of its total charge the first day and up to 4 percent of its charge every week hence. Sure, some LSD (low self-discharge) variants of the NiMH formula are available but cut back capacity by 20 percent in order to do so. NiMH batteries also have a reputation for being heat sensitive which drastically shortens their operational lives.

How to Get the Most Out of Your Nickel-Based Batteries

Sealed nickel batteries are the most common form for consumers and don't require much in the way of general maintenance to continue operating. However, there is still much you can do to keep it operating at peak performance. NiCd batteries, for example, can be charged at varying rates depending on the cell's construction, ranging from 14 hours down to just 10 minutes. However, as the amperehour rating increases, so too does the internal resistance (which generates heat and slows the electron transfer) and the chances of overcharging (which again generates heat and can actually damage the cell and shorten its operational life). Since NiCd batteries will heat up more as they near full capacity, you should invest in a compatible charger with an temperature gauge automatically shuts off the current flow when the battery heats to a certain degree, preventing an over-charge.

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NiCd batteries also tend to self-discharge, though not to the degree of NiMH cells—typically just 10-20 percent a month, depending on the temperature. As such, if you need to store an NiCd battery for long periods, you'd do well to first run it down to below a 40 percent charge before placing it in a cool spot. Some battery makers even recommend fully expending the battery and then short circuiting the leads for long term storage as well, so make sure you check the mfg website for additional information before doing so. Even when they are not about to be packed away, NiCd batteries (and all dry cells for that matter) do well with the occasional full discharge in order to prevent memory effect—once every 30 cycles or so should do it. If these steps are followed, NiCd batteries can last as long as five years in storage. NiMH batteries, on the other hand, will last about three years but do require priming beforehand.

Most Lithium-ion batteries, such as those powering the laptop or tablet you're likely reading this on, employ a simple carbon anode and a highly-conductive electrolyte mix of ethylene carbonate or diethyl carbonate. They differ from their non rechargeable cousins, however, in that Li-ion batteries are capable of reversing their chemical reactions thanks to a cathode made from lithium cobalt oxide. Intercalation is defined as "to insert between or among existing elements or layers" by Merriam Webster. Essentially, the anode of a rechargeable Li-ion are made from many millions of microscopically thin sheets of carbon graphite, stacked atop one another. When these batteries discharge, the positive lithium ions are transported from the anode and deposited into the cathode's matrix while the cobalt is oxidized. When charging the battery, the process is reversed. The cobalt is reduced and lithium ions return to the anode.

The metallic lithium battery that Whittingham developed in the 1970s provided an impressive energy density and, throughout the 1980s, many companies attempted to develop a rechargeable version based on the same chemistry. Unfortunately, lithium is an inherently unstable metal and is prone to thermal runaway—that's when flaming gasses forcefully exit the battery—when overcharged. The internal resistance would rise so quickly and generate so much heat that the lithium in the cell would melt and routinely exploded. During a thermal runaway, the intense heat of the venting cell will spread to the next cell, setting off a chain reaction. Recalls on these early cells were common throughout the 1980s, 1990s and first decade of the 21st century. As a result, researchers replaced the volatile metallic lithium with a more stable blend of material containing lithium ions. John Goodenough and K Mizushima are credited with creating the first rechargeable lithium in 1979 when they successfully demonstrated a cell using the same lithium cobalt oxide (LCO) formula that is primarily used today. Sony introduced the very first commercial Li-ion battery in 1991. Lithium ion batteries typically take one of four form factors: small cylinder, such as those powering your laptop; large cylinder, which are not widely available; pouch, which are soft-bodied and offer the highest energy density of the four (a big reason they're often found in cell phones); and prismatic, cells stored in semi-hard plastic cases and often employed in automotive electrical systems.

LCO batteries offer numerous advantages over nickel-based cells. Lithium cells are smaller, lighter, more energetically dense, and able to operate within a much wider temperature range than other rechargeable—essential features in high-tech, high-drain, gadgets and mobile devices. What's more, lithium-ion batteries have a much lower self-discharge rate than other secondary batteries. LCO's lose just 5-10 percent of their charge per month, compared to more than 30 percent a month for NiMH cells and 10 percent a month for NiCd batteries.

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They do however suffer from some significant shortcomings as well. The rechargeable lithium chemistry is more expensive to make—and recycle—than NiCd. In addition, there is no shortage of ways that Lithium ion batteries can fail, especially LCOs—overcharging them results in thermal runaway and potentially catastrophic failure, deep discharging them causes short circuits. To combat this problem, all Lithium ion batteries are equipped with a protective heat-sensitive limiter circuit that prevents the battery from charging or discharging too quickly. Unfortunately, this circuitry also results in a relatively low discharge rate and continually draws a small amount of current which accelerates its self discharge rate. They also tend to build internal resistance as they age, rendering the battery useless after a few years or few hundred cycles.

Since the cobalt component of LCO lithium adds so much to the battery's production and recycling costs, John Goodenough's research group at the University of Texas in 1996 developed an alternate chemistry utlizing lithium iron phosphate (LiFePO4 or LFP) as its cathode rather than cobalt oxide. While these batteries don't have quite the energy density of their cobalt cousins, LFP is an inherently more stable chemistry with stronger chemical bonds than those binding Cobalt and Oxygen. Its iron-rich composition makes for a more thermally stable cell with a longer operational lifespan and higher discharge rate than cobalt-based lithium. Plus, that its made from non-toxic, readily available materials, makes it easier and 14 percent cheaper to produce than sLCOs. As such, LFP batteries are more likely to be found in automobiles made by Aptera and QUICC as well as a host of boutique electric motorcycle manufacturers, industrial power applications than modern gadgets. One notable exception is the One Laptop per Child project which chose LFPs because their non-toxic nature complies with the European Union's Restriction of Hazardous Substances Directive.

The third form of Li-Ion battery uses lithium manganese oxide for its cathode. Developed in 1996, this chemistry has an inherently high thermal stability—able to discharge at 20-30 amps without getting hot—and low internal resistance, which not only makes it the safest and most stable Lithium-ion chemistry, it is also ideal for fast recharging, high-current discharging applications—Power tools, e-bikes, electric vehicles, and medical medical devices—and eliminates much of the safety needed by cobalt-based cells. The main drawback of lithium manganese oxide is its paltry energy capacity compared to LCOs. A 18650 size manganese battery typically packs about 1200mAh of power—that's barely half of what a similarly sized LCO can hold.

How to Get the Most Out of Your Lithium-Ion Batteries

Under normal usage conditions, a Li-ion battery will last between three and five years before internal resistance builds high enough to render its cells useless. This aging effect is governed not only by the number of charging cycles but also the ambient temperature.

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Luckily, minimizing this occurrence and extending the serviceable life of your battery requires little more than keeping it cool and fully charged. Since the resistance build-up gets worse the more the battery discharges and rapidly degrades the cell's capacity, regularly topping off the charge is the easiest way to maximize your battery's operational life. That's not to say the battery should perpetually be kept at its maximum charge voltage, quite the opposite in fact—all secondary batteries outside of the lead acid variety require a period of rest after charging to prevent undue stress.

Low power isn't the only condition that harms li-ion batteries, elevated temperatures can be even more damaging. In fact, leaving a cell phone on a hot dashboard or keeping a laptop plugged into the grid for extended periods of time will cause cell oxidation, drastically shrinking the battery's capacity and shortening its lifespan to as little as 48 months. Therefore, lithium ions should only be stored and used at room temperature.

For long-term storage a little charge goes a long way for li-ion batteries. Unlike lead-acid cells which demand a trickle charger to maintain their full voltage levels, lithium and nickel based batteries actually store better on a partial charge. Many manufacturers recommend draining the battery to 40 percent if you plan on leaving it on a shelf for more than a few months. This level allows for a reasonable degree of self-discharge while maintaining enough power in reserve to keep the protection circuit powered.

Like the open bar at your sister's wedding, devices slurping from the public power grid taps have little impetus to exercise restraint in their consumption. High-end gaming systems now sport 1600W power supplies driving walls of monitors and multiple multi-core processors, desktop towers never bother sleeping or darkening their displays or shutting down unused system components because, with an effectively unlimited resource supply, why bother conserving?

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Battery-powered devices, on the other hand, must take a more miserly approach with their limited power supplies by activating hardware components like the GPS, Bluetooth, or Wi-Fi antenna only when needed and entering sleep mode whenever possible. At least that's how it's supposed to work. Problem is that if these components are left on, they'll regularly search for possible connections, preventing the device from sleeping and rapidly draining the battery. The device's display is also culpable in your battery's untimely demise. Left on the highest brightness setting, the screen can account for upwards of 80 percent (or more) of a device's power draw. Turning off Wi-Fi and Bluetooth when they aren't actively being used, using the CDMA protocol whenever possible (since LTE powers two antennae to boost throughput), and setting the screen to its lowest readable brightness and shortest available screen timeout will all help keep the device's power consumption in check.

The device hardware isn't solely to blame mind you, the same can be said for the installed software. Data auto-fetching, for example, be it a twitter stream, email client, or Facebook timeline, will draw power every time it pings for new data. Luckily, most apps allow you to determine the rate at which they check for updates, so set them for the longest interval available or turn off push notifications altogether and just manually sync when you're actually using the app.

Apps subsidized by third party ads may save you a few bucks, but they suck down an inordinate amount of power as they continually ping remote servers to download and refresh the displayed banners. In fact, a recent study conducted by Purdue University and Microsoft found that apps supported by third party ads can spend as much as 75 percent of their power draw driving ads—Angry Birds, inj some cases, allots as little as 20 percent of the electricity it draws on actual gameplay.

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Streaming media—video especially—games, and other graphic-intensive functions are a huge power draw. When streaming, not only is the device expending energy to maintain its network connection to import the data, the CPU is going full-tilt to process the incoming information and simultaneously drive playback—while the display and speakers both draw additional power and processing resources. This is hard, labor-intensive work for a mobile device to be sure. So instead of streaming, download your media over a Wi-Fi connection to local storage. This not only demands less energy than a mobile data connection, it also preserves your monthly data allowance.

System applications—whether its the preinstalled email client, remote backup, or navigation apps—may also be guilty of excessive power use. You can find out which apps and processes are hogging power through your device's power manager. On Android systems, open Settings and select Battery for a comprehensive list of every feature currently drawing current. For iOS, select Settings on the Home screen and choose General, then Usage. WP owners can find the current status of their device's battery from the Battery Saver menu under Settings.

The Future of Batteries

Lithium-ion has become the dominant chemistry of rechargeable batteries thanks to its relatively high energy density. But as emerging applications for li-ion, such as "smart" utilities and electric vehicles, continue to grow, the chemistries we use today won't be able to keep up tomorrow. As such, researchers are already hard at work developing exotic metallic mixtures like Lithium-Silicon whose volume swells 300 percent on a full charge and shrinks back as it drains. As Gao Liu of Berkeley Lab's Environmental Energy Technologies Division (EETD) explains in a statement,

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"Most of today's lithium-ion batteries have anodes made of graphite, which is electrically conducting and expands only modestly when housing the ions between its graphene layers. Silicon can store 10 times more – it has by far the highest capacity among lithium-ion storage materials – but it swells to more than three times its volume when fully charged."

Lithium-air batteries, which oxidize lithium while reducing oxygen to generate current, were initially devised in the 1970s but have just recently become feasible thanks to recent advances in related material sciences. These batteries boast a massive energy density potentially able to rival internal combustion engines. Other researchers are already looking beyond the battery to alternative technologies such as ultracapacitors, which "store energy with a static charge that is resonant over a large surface area of material in the construct of the product," Ioxus CEO Mark McGough explained to Green Tech Media, and radically divergent chemistries like the PolyPlus battery which utilizes the oxygen in tap water to draw lithium ions across a circuit—it's a battery that only works when wet.